Advances in measurements of unsaturated soils

1,2Colin S. Campbell, 1Gaylon S. Campbell, 1,2Douglas R. Cobos, and

1Bryan T. Wacker1Decagon Devices, Inc., Pullman, WA

2Washington State University, Pullman, WA

Characterizing unsaturated soilsRelationship between water potential and

water content defines soil water characteristic curve (SWCC)Soil water characteristic curve (SWCC) is

central to the behavior of unsaturated soils (Fredlund and Rahandjo, 1993; Barbour, 1998)

Key in understanding unsaturated soils likeCompacted soilsSwelling claysLow bulk density soils

Characterizing unsaturated soilsMeasurements

Water content is relatively easy to measure

Suction requires more sophisticated and time-consuming methods

GoalInvestigate two improved methods

for obtaining SWCC

Background: Creating the soil water characteristic curve

Soil water contentSoil suction Soil suction

Background: Filter Paper Filter paper method for suction measurement

Based on work by Hamblin (1981), Al-Khafaf and Hanks (1974), and Deka et al. (1995)

Calibrated method Measured water content of filter paper Correlated with suction

Standard vapor pressure method and tensiometer Generated SWCC

Provided suction measurements without difficult lab setup

Problems Calibrated method that relies on repeatable SWCC Results are affected by equilibration time, hydraulic

conductivity, paper contact with soil, fungal growth Filter paper SWCC has hysteresis Labor and time intensive

Background: Pressure Plate Introduced in 1930s by L.A. Richards

Equilibrate pressure above soil sample with water in sample

Forces water out of the sample so soil suction is equivalent to air pressure

Equilibration times Wet samples (up to 100 kPa suction) < 1 day Dry sample (100 kPa to 1500 kPa): 1 week to

never Problem

Range from 100 kPa to 1000 kPa important to SWCC

Axis Translation may not ever fully equilibrate in upper range (Bittelli and Flury, 2008)

“No-man’s Land” of suction

instrumentation

Measurement MethodsEvaluate SWCC

Liquid equilibrium for wet regionTensiometerWIND/SCHINDLER integrated tensiometer

and scale evaporation methodVapor pressure method for dry region

Simple, fast (5 to 15 min)Evaluate consistency between wet

and dry regions

Tensiometer: Suction in “wet” soilEquilibrates water under

tension with soil water through a porous cup

Measures pressure of water

Highest accuracy, but limited range (Suction: 0 to 80 kPa)

Must be measured in representative sample (compaction)

“Wet” Soil SWCC using WIND/SCHINDLER Evaporation Method

Suction in “Dry” range

Cool mirror until dew forms

Detect dew optically Measure mirror

temperature Measure sample

temperature with IR thermometer

Suction is approximately linearly related to Ts - Td

Infrared SensorMirror

Optical Sensor

Fan

Sample

Let stand 24 hAdd water

Mix

Fill sample cup

Reading the Suction

Insert sample Seal chamber

Wait 5-10 min. and read the result (up to 15 min for very wet samples)

0.001

0.01

0.1

1

10

100

0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350

Wat

er p

oten

tial (

-Mpa

)

VWC (m3/m3)

all

dry range

wet range

Silt loam SWCC: Tensiometer & WP4

Data Void: Original WP4Suct

ion

(MPa

)

Water Content (g/g)

New WP4C: 10x better temperature

measurement: 0.001o C precision

Results

Chilled mirror absolute error of wet-end suction (WP4C and WP4)

Error of Original Chilled Mirror Sensor: WP4

Soils

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #1

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000

grav

imet

ric w

ater

cont

ent

(g/g

)

water potential (-kPa)

Soil B2

WP4C dewpoint

T5 tensiometer

Campbell and Shiozawa

Suction(kPa)

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #2

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000

grav

imet

ric w

ater

cont

ent

(g/g

)

water potential (-kPa)

Soil B4

WP4C dewpoint

T5 tensiometer

Campbell and Shiozawa

Suction(kPa)

Schwana loamy fine sand

Kiona fine sandy loam

Palouse silt loam

WIND/SHINDLER measurements

Wolfgang Durner, Personal Communication

WIND/SHINDLER SWCC and hydraulic conductivity function

Wolfgang Durner, Personal Communication

Summary New techniques make determining soil water

characteristic curves easier and more accurate Improved measurement range Faster and less time consuming measurements

New chilled mirror measurements bridge traditional “no man’s land” Measurements at low suctions match nicely with

tensiometer WIND/SCHINDLER method allows automation of “wet”

range SWCC and unsaturated hydraulic conductivity Simple drying procedure Software fits SWCC and gives hydraulic conductivity

function

References Al-Khafaf, S., and Hanks, R.J. 1974. Evaluation of the filter paper

method for estimation soil water potential. Soil Sci. 117:194-199 Barbour, S.L. 1998. Nineteen Canadian geotechnical colloquium:

The soil-water characteristic cure: A historical perspective. Canadian Geotechnical Journal. 35:873-894.

Bittelli, M. and Flury, M. 2008. Errors in Water Retention Curves Determined with Pressure Plates. Soil Sci. Soc. Am. J. 73:1453-1460

Deka, R.N., Wairiu, M., Mtakwa, P.W., Mullins, C.E., Veenendaal, E.M., and Townsend, J. 2995. Use and accuracy of the filter-paper technique for measurement of soil matric potential. Eur. J. Soil Sci. 46:233-238

Fredlund, D.G. and Rahardjo, H. 1993. Soil mechanics for unsaturated soils. John Wiley and Sons, Inc.: New York.

Gardner, W.R. 1937. A method of measuring the capillary tension of soil moisture over a wide moisture range. Soil Science. 43(4), 277-283

Hamblin, A.P. 1981. Filter paper method for routine measurement of field water potential. J. Hydrol. 53:355-360